Carbon black exhibits hydrophobicity and low wettability with water. Therefore, it is difficult to stably disperse carbon black in water at high concentrations. This is because the surface of carbon black has only a small number of functional groups (e.g. hydrophilic hydrogen-containing functional groups such as a carboxyl group and a hydroxyl group) having a high affinity with water molecules. Various methods have been proposed to improve the dispersibility of carbon black in water by oxidizing the carbon black to produce hydrophilic functional groups on the surface of the carbon black.
For example, JP-A-48-018186 discloses a method which oxidizes carbon black using a hypohalite aqueous solution, and JP-A-57-159856 discloses a method which oxidizes carbon black using a low-temperature oxygen plasma.
A method of producing an aqueous ink has also been proposed in which the dispersibility of moderately oxidized carbon black in water is improved using a coupling agent, a surfactant, or the like (e.g., JP-A-4-189877). However, it is difficult to stably maintain the dispersibility of the carbon black for a long time since the surfactant or the like is oxidized or decomposed due to a change in temperature and a change with time.
JP-A-8-003498 discloses a water-based pigment ink including water and carbon black, wherein the carbon black has a surface active hydrogen content of 1.5 mmol/g or more, and a method of producing a water-based pigment ink including water and carbon black comprising (a) providing acidic carbon black and (b) further oxidizing the acidic carbon black in water using a hypohalite. JP-A-8-319444 discloses a method of producing a water-based pigment ink including finely dispersing carbon black with an oil absorption of 100 ml/100 g or less in an aqueous medium and oxidizing the carbon black using a hypohalite.
According to the methods disclosed in JP-A-8-003498 and JP-A-8-319444, a water-based pigment ink with excellent water-dispersibililty and long-term dispersion stability is obtained by oxidizing carbon black so that the surface of the carbon black has a large number of active hydrogens (i.e. hydrophilic functional group).
On the other hand, the number of hydrophilic functional groups existing at the contact interface between the surface of the carbon black particles and water molecules is important for dispersing the carbon black in water to maintain a stable dispersed state. Therefore, it is difficult to accurately determine the dispersibility of the carbon black merely based on the number of functional groups per unit weight of carbon black. When using a method which finely atomizes carbon black in water using glass beads and oxidizes the carbon black using a hypohalite to treat the surface of the carbon black while improving its dispersibility, the atomization effect is diminished due to the buoyancy applied to the glass beads in water. Moreover, since active sites are rarely formed, it is difficult to uniformly form functional groups on the surface of the carbon black.
JP-A-11-148027 discloses a water-dispersible carbon black which is modified by oxidation and in which the total amount of carboxyl groups and hydroxyl groups among the hydrogen-containing functional groups present on the surface of the carbon black is 3 μeq/m2 or more per unit surface area.
JP-A-9-286938 discloses a water-based pigment ink in which carbon black is oxidized with a hypohalous acid and/or a hypohalite and at least some acidic groups existing on the surface of the oxidized carbon black are bonded to an amine compound to form an ammonium salt.
In JP-A-9-286938, ammonia, a volatile amine replaced by an alkyl group having 1 to 3 carbon atoms, an amine compound replaced by an alkanol group (i.e., alkanolamine or alkylalkanolamine), or the like is used as the amine compound. Since the compounds other than ammonia have a low degree of dissociation, a substitution reaction with hydrogen in the acidic group does not easily occur.